This invention relates to a coil device, and in particular, to a coil device used as the helical coil, the toroidal coil, the poloidal coil, the charged particle accelerator, etc. of a fusion experiment apparatus.
FIGS. 1 to 4 show examples of conventional coil devices, in particular with respect to the structure of the current feeding sections thereof. FIG. 1 shows the structure of a standard-type coil. In the structure shown in FIG. 2, the thickness of the coil feeder 3 is less than that of the coil conductor 2 so that the error magnetic field generated by the coil feeder can be diminished. The structures shown in FIGS. 3 and 4, which are disclosed in Japanese Patent Laid-Open No. 53-134405, are improvements of those shown in FIGS. 1 and 2. In these drawings, the coil 1 is composed of coil conductors 2 and feeder 3. Electric current flows in the direction indicated by the arrow 4. The feeder 3 includes in the corner thereof slits 5. In order to generate the required magnetic field, electric current is passed through the coil conductor 2 from the feeder 3.
When performing such operations as the trapping or controlling of plasma, or the deflecting of charged particles using the magnetic field generated by a conventional coil device of this type, any "disturbance in the magnetic field", which is generally called an error magnetic field, can have a bad influence on the trapping or controlling of plasma or on the deflection of charged particles. It is consequently necessary to diminish this error magnetic field as much as possible. The winding and arrangement of the coil conductor must accordingly be very accurate. Moreover, this makes it necessary to prevent the magnetic field generated by the feeder lead section from having a bad influence on the normal magnetic field generated by the coil conductor section.
In the case of the structure shown in FIG. 1, the electric current required for generating the magnetic field is a circular current. When the ideal circular current is removed from the actual current including the current flowing through the feeders 3, the current component shown by the closed curves (a) in FIG. 5 remains. This current component constitutes the error magnetic field current loop which causes an error magnetic field to be generated.
The conventional coil device shown in FIG. 1 involves a large error magnetic field loop, which can have a bad influence on such device functions as plasma trapping. The structure shown in FIG. 2, which is an improvement of the structure shown in FIG. 1, also involves a similar error field current loop, so that it cannot be applied to a device of which a high magnetic field accuracy is required.
In the feeder 3 of the structure shown in FIGS. 3 and 4, in a top and a perspective view, respectively, the current distribution adjustment is effected by means of slits 5. As shown in FIG. 6, the error magnetic field current loops generated in this structure are relatively small. Further, these current loops generaged are in the directions opposite to each other (i.e., one is clockwise and the other counterclockwise), thus canceling each other. However, electric current is concentrated in the slit sections 5 of the feeder section, raising the temperature of these slit sections due to the Joule effect. In addition, the strength of this structure is exceedingly inadequate because of the slits 5, so that it cannot be applied to a coil device on which great magnetic force is exerted.